This invention relates to a filter circuit comprising an amplifier including an amplifier element having an input electrode coupled to an input terminal for receiving an input voltage and an output electrode coupled to a terminal of a power supply source, a capacitor coupled to the amplifier and an output terminal coupled to the capacitor for making an output voltage available.
In a generally, known circuit the amplifier element, for example, a transistor forms a part of a differential amplifier. In operation the voltage across the capacitor is continuously compared with the input voltage. The capacitor is charged or discharged by an output current of the differential amplifier, which current is dependent on the difference between the input voltages. In a balanced state of the circuit the voltage across the capacitor is substantially equal to the input voltage and the output current is substantially zero, with approximately as much charge being applied to the capacitor as is withdrawn from it. If this state is disturbed for some reason, so that the said voltages become unequal, it is restored after some time. This time depends on the capacitance of the capacitor and on the amplification of the differential amplifier. It is apparent therefrom that the circuit has a time constant and behaves as a low-pass filter.
Another generally known circuit consists of a peak detector in which a capacitor is charged by the peak values of the input voltage of the circuit, whereafter it is discharged. Here, too, a time constant is important.
If a comparatively large time constant is desired for the circuit, the capacitor should have a considerable capacitance. However, the capacitance may be kept small so that the capacitor can be incorporated in an integrated circuit comprising the other components of the circuit if the charge and discharge currents of the capacitor are small. For this purpose circuits have been proposed which use current mirror circuits having a more or less complicated structure in which a current of moderate strength is converted into a very small current. These circuits require a considerable surface area in the integrated circuit.